A known solution for generation of a constant current provides for the use of an appropriately biased MOS transistor.
Indeed, by applying a biasing voltage between its gate and source terminals, the MOS transistor is caused to conduct a constant current between the source and drain terminals.
As known, there exists a biasing voltage V.sub.gs =V.sub.gsx of the gate of an MOS transistor for which the drain current is constant with temperature variation: EQU I.sub.D =.mu..multidot.C.sub.OX .multidot.(V.sub.gs -V.sub.th).sup.2(1.1)
Where .mu. is the mobility of electrons, C.sub.OX is the capacitance of the silicon oxide, V.sub.gs is the gate biasing voltage and V.sub.th is the threshold voltage of the MOS transistor.
This relationship can be readily deduced by observing the drain current I.sub.D as a function of the V.sub.gs with different temperatures as illustrated in FIG. 1 which shows the current-voltage characteristics of an MOS transistor with three different temperatures T1, T2 and T3.
As may be seen in FIG. 1, there is a point on the chart corresponding to a voltage V.sub.gsx at which the three curves intersect. This relationship leads to the assumption that by using this current to charge a capacitor there could be provided an electrical signal of constant duration with temperature change of the device.
In reality the problem is not so simple since C.sub.OX, V.sub.th and .mu. vary with the process in addition to varying with the temperature.
The mobility of electrons .mu. varies very little with the process because it is one of the best-controlled parameters and, indeed, it depends mainly on the doping element and is known with an accuracy of greater than 95%, the mobility can thus be considered dependent on temperature alone in a first approximation.
The problem goes back to compensating for the error introduced by the variation in the gate oxide thickness and the threshold voltage.
The prior art eliminates dependence on C.sub.OX by using as capacitance a capacitor whose dielectric is the same gate oxide used in the transistors. In this manner the relationship between MOS current and capacitance becomes: ##EQU1## where K.sub.1 is a constant area factor.
A known circuit diagram which permits providing an electrical signal by this method is shown in FIG. 2.
In FIG. 2 a capacitor C is connected between a ground reference voltage GND and a constant current generator consisting essentially of an MOS transistor M1 biased with a voltage V.sub.gsx between the gate and source terminals. The voltage present on the capacitor C is applied to a first input terminal of a voltage comparator COMP while a second input thereof is connected to a reference voltage V.sub.ref. The comparator COMP then compares the voltage at the terminals of the capacitor with the reference V.sub.ref and supplies at output a logical signal which is the result of the comparison.
One disadvantage of this circuit is that the I.sub.D /C.sub.OX ratio is strongly dependent upon the threshold voltage of the transistor M1 since a variation of the threshold causes the transistor being no longer correctly biased. Consequently the I.sub.D /C.sub.OX ratio also varies with temperature.